So it's that time again for me to renew my Cisco Certifications. As a result, this post is based on my preparation for the CCNP Route Exam (300-101).
In this post I will be focusing on Advanced EIGRP topics
- Advanced Distance Vector
- Fast Convergence
- Loop Free
- Uses multicast NOT broadcast
EIGRP uses 3 branches of logic
- Neighbor relationship which are listed in the neighbor table
- Topology information is exchange with neighbors in the table
- Use the topology table to find the best route
EIGRP Topology Table
- Basic information used - prefix, prefix length, metric information, other details
- Learned topology information is stored in the topology table
- Topology table is analyzed to determine the best route
- Topology data must exist before it can be sent to the neighbor
- The local EIGRP process adds networks to its local topology via:
- locally connected interfaces with EIGRP enabled
- interfaces referenced in the "neighbor" command
- redistribution of routes into EIGRP
- Information is first added to the local topology table before being sent to neighbors
- Hop count of "0" means the route is locally connected
5 EIGRP Packets
- Hello
- Update neighbor relationships
- sends every 5 seconds by default on broadcast links, ethernet, etc.
- sends every 60 seconds by default on Frame Relay, etc
- Sourced from the primary interface
- does not require acknowledgement
- Update
- Used as part of the data exchange process
- Send routing updates
- contains topology information
- contains:
prefix
prefix length
metric components (bandwidth, delay, reliability and load)
non metric components: MTU and hop count
- requires acknowledgement
- Query
- used when moving a route from passive to active
- Ask neighbor about routing information
- Requires acknowledgement
- Sent out all interfaces except the interface of the successor
- if the neighbour does not have the lost routes, it then sends out queries to its neighbors
- Reply
- Reply to query about routing information
- Requires acknowledgement
- Ack
- Used as part of the data exchange process
- Acknowledges the update packet
- Ensures reliability
- does not require acknowledgement
- The "delay" command uses units of tens-of-microseconds.
- "show ip eigrp" and "show interfaces" command uses microseconds
EIGRP update process
- Topology information is exchange via the update packets
- future updates are only sent as a result of some type of change in the network or the EIGRP topology, etc
- if neighbor fail and recover, full adjacencies are formed
- uses split horizon by default, limiting the prefixes advertised out an interface
- uses Reliable Transport Protocol (RTP) to send updates and confirm receipt
- On LAN updates are sent via multicast (224.0.0.10). ACK via unicast
Concerns for EIGRP and WAN - Frame Relay
- Neigbors must be reachable via Layer 2 before Hello messages can be exchanged
- Split-horizon must be disable on the interface (no ip split-horizon eigrp 10)
EIGRP - Controlling WAN Bandwidth
- By default only 50% of the bandwidth assigned to the interface is used with the bandwdith command
- Bandwdith can be changed by using "ip bandwidth-percent eigrp 10"
- Messages are queued in memory
- default bandwidth on serial links is 1544 kbps
- Bandwidth must be set on both physcal interfaces and sub-interfaces
- Set the bandwidth on point-to-point links the same as the Committed Information Rate (CIR)
Getting the best route
- Metric is calculated from the perspective ofo both the local router and its neighbor
- Feasible Distance (FD) - Used by local router to determine the best route from its perspective
- Best route based on lowest metric (FD), minimum distance
- Successor router is the next hop router
- Reported Distance (RD) - Metric from the neighbor's perspective. Used by local router when converging
- Reported by upstream neighbor
- Advertised Distance is the same as Reported Distance
- Used when falling back to an alternative route
- used to determine if a feasible successor is loop free
- Successor route is placed in the routing table
- Has the least cost to a destination
- next hop for forwarding packets
- load balancing allows multiple successors
- Feasible Successor
- backup/alternative routes
- if no feasible successor is exist, reccompute new successor
- becomes a feasible successor if the reported distance is lower than the feasible distance
- Metric calculation grows as the bandwidth decreases. Slower bandwdith results in worse metric
Some methods of Tuning EIGRP
- Setting bandwidth
- uses kilobits per second (kbps)
- Can affect other metrics
- if having to choose between bandwidth and delay, choose delay
- Bandwdith command is used for other tasks on the router (SNMP, etc), delay is not as much
- On LAN interfaces bandwdith and delay default to the speed of the interface
- setting interface delay
- uses tens-of-microseconds
- changing metric calculation - K-values
- can be modified as needed
- valid values are 0-255
- by default K1 = K3 = K1, others default to 0
K1 = 1, K2 = 0, K3 = 1, K4 = K5 = 0
- K-values must match before routers can become neighbor
- Cisco recommends not to use K2, K4 or K5. Non zero value causes the devices to use load and reliability
- Load and reliability contribute to route flapping
- offset lists
- allows the additions of values to the calculated metric of a route
- first create an EIGRP offset list that define the value to add
- uses IP ACL to permit traffic
- Match direction of update messages
- match interface for received or sent update
- set interger metric for FD and RD
- Two important parts are ACL and offset-list
#access-list 15 permit 10.0.0.0
#router eigrp 10
#offset-list 15 in 10 fa2/0
Load sharing
- convergence from successor to feasable successor should occur within a second
- multiple routes can be placed into a routing table for a specific network
- routes are then balanced by destinations (default)
- convergence occurs faster
- uses "maximum-paths" or "variance" command
- "Show IP EIGRP topology" only shows successor and feasibile successor
- "Show IP EIGRP topology all-links" list ALL routes
Going Active
- Process of discovering loop free alternatives when there are no feasible successor
- routes move from passive to active when their is no feasible successor
- Sends Query Messages to each neighbor except the neighbor in the failed route
- if the neighbor has a passive route for the network in question, it is considered loop free
- neighbor send Reply
- Passive routes have successor information
Passive routes are good routes
- Active routes have lost their successor. No feasible successor is available. The router is actively looking for new routes.
Active Routes are bad routes
Stuck-in-Active
- Routers may wait a while before getting a Reply messages
- active timer (3 minutes default) is used to determine how long a router must wait for a Reply Messages
- When a router does not receive a Reply within the active timer, it is considered stuck-in-active
- Stuck-in-active is not the state you would like your router to be in
Stub Routers
- Stub routers should not forward traffic between two remote EIGRP learned subnets
- uses "eigrp stub" command (default and uses connected and summary routes)
- eigrp stub options [ connected, summary, static, leak-map, redistributed, receive-only ]
- stub routers do not advertise routes learned from one neighbor to another
- non-stub routers keeps track of stub routers
- non-stub routers do no send query messages to none stub routers
- will form neighborship even in "receive-only" mode
- Queries are suppressed on neighbors
Router Filtering
- Uses ACLs
access-list 3 deny 172.16.0.0
router eigrp 10
distribute-list 3 out
- Or prefix-list
- Allows matching of prefix length
- uses a tree structure
- matches route prefix and route subnet
- "ip prefix-list" command
ip prefix-list SecurityNik seq 5 deny 172.16.0.0/24
router eigrp 10
distribute-list prefix SecurityNik out
- Route maps
- uses "distribute-list"
- a route map command uses one ore more route map
- commands are processed in sequential numbers
access-list 3 deny 172.16.0.0
route-map SecurityNik deny 10
match ip address 3
router eigrp 10
distribute-list route-map SecurityNik out
- match command can use ACL or prefix-list
- use "show route-map" to validate config
Route Summarisation
- used to keep routing tables small and manageable
- conserves memory
- can be done on any router
- Reduces Query Scope
- sub-optimal routing is possible
- uses "ip summary-address eigrp asn prefix subnet-mask"
causes route to go down then backup
- Local summary route uses an administrative distance (AD) of 5
- manual route sumarisation or auto summarisation
- routers can auto-summarised
- summary is created for each classful network
- classful protocols like RIPv1 and IGRP) cannot support VLSM
- Default route can be considered the best summary route
- Auto summarisation is on by default
EIGRP Authentication
- Used to authenticate every EIGRP messages
- uses pre-shared key
- a MD5 digest is generated for each message based on the pre-shared key
- Messages are silently discarded if the digest does not match
- Authentication must be successful before neighborship can be established
- helps to prevent DOS
- does not provide privacy
- First create key chain
(config)#key chain SecurityNik
(config-keychain)#key 1
(config-keychain-key)#key-string EIGRP-Auth
(config-if)#ip authentication mode eigrp 10 md5
(config-if)#ip authentication key-chain eigrp 10 SecurityNik
- enable MD5 authenticate on the interface
- use the key chain previously created on the interface
Reference:
CCNP official Cert Guide - CCNP Routing and Switching - Route 300-101
http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/16406-eigrp-toc.html
http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfeigrp.html
http://www.cisco.com/c/dam/en/us/products/collateral/ios-nx-os-software/enhanced-interior-gateway-routing-protocol-eigrp/prod_presentation0900aecd80310f03.pdf
http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/enhanced-interior-gateway-routing-protocol-eigrp/whitepaper_C11-720525.html
http://www.cisco.com/c/dam/en/us/products/collateral/ios-nx-os-software/enhanced-interior-gateway-routing-protocol-eigrp/Advances_In_EIGRP.pdf
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