CCIE L3 Ep.12 3nh4nc3d_dual.7z
Eenhanced Interior Gateway Protocol is a distance-vector protocol with classless routing capabilities that was propriety from Cisco. Maybe some others can learn how EIGRP works (aka RFC information RFC7868).
Note, distance vector knows that connected directed neighbor tell them. That’s the opposite of link-state protocols (OSPF and IS-IS).
By default, EIGRP uses bandwidth and delay for metric (composite metric) calculation.
The specific algorithm used is called "DUAL", a Diffusing Update Algorithm as referenced in "Loop-Free Routing Using Diffusing Computations" - rfc7868
Advantages
. The first advantage behind EIGRP is the possibility to compute the best loop-free path and its backup loop-free path — fast convergence.
. Another benefit is permission to make filtering and summarization where is the best for network administrators.
. Unequal cost load balance — In theory unequal cost load balancing is good. But real-world applications maybe cannot deal with it. Some flow of applications from different links might result in in-app problems.
Terminology
Some biography discusses “principal” keywords. Be careful, access RFC 7868, and deep dive into the DUAL algorithm.
Autonomous Systems (AS):
A subset of routers and routing exchanging of a single entity.
Downstream Router
A router that advertises into routing protocol the target route (network).
Upstream Router
Router closest to the origin traffic ( example: Router that is the gateway of the source IP address that creates the specific traffic).
Neighbor/peer
A neighbor is a device that creates adjacency with another device running EIGRP.
Successor
The best least-cost path for a particular destination.
Feasible Successor
A route reported by neighbors that meet the feasibility condition for a particular destination.
Reported Distance
Distance reported by neighbors for each network
Computed distance
Least-known total metric to a destination from the local router.
Feasible Distance
A record of last least known total metric of the last Active-to-passive event
Feasible condition
If the neighbor’s RD is less than this router’s FD the feasible condition is met. FD says that the neighbor’s RD provides a loop-free path to a destination. The only route that meets feasible conditions can be used for unequal load balance.
Route States
Route to a destination can be in a PASSIVE or ACTIVE state:
ACTIVE
Route reported by a neighbor that doesn’t pass the Feasibility condition check. Re-computation is required. The route cannot be installed in the RIB.
PASSIVE
The feasibility condition is met and the route can be installed in the RIB. This is a desirable state.
EIGRP Packets
EIGRP packets are directly encapsulated into a network layer with protocol ID 88, both IPv4 and IPv6. If I am not wrong, IPX has supported it too.
By default, EIGRP limits itself to using no more than 50% of the bandwidth of the interfaces running EIGRP.
- Hello - Maintenance neighbor relationship. Every 5secs | Hold time 15 (three hello’s time). For low-speed (T1) circuits … Hello 90sec and Hold 180sec.
- Update - Report new metric status or add/remove the route.
- Query - When reported distance doesn’t meet FC, a query message is created to find a loop-free path to the specific network
- Reply - Response Query message
- SIA-Query- When the reply message takes a long time to run, a new query called SIA query is sent.
- SIA-Reply - Respond SIA-QUERY.
Each packet will use either unicast or multicast network destination:
224.0.0.10 for IPv4 “EIGRP Routers”
FF02::A for IPv6 “EIGRP Routers”
In IPv4, the source IP address used is what is set in the egress interface. Another way, for IPv6, is the local IP address set in the egress interface.
STUCK IN ACTIVE (SIA) is when a routing transit to an ACTIVE state for a particular destination. a Query message is sent to a neighbor(all neneighborsxcept for stub routers) and the ACTIVE timer is started to limit the amount of time a destination may remain in an ACTIVE state (default is 3min).
SIA-QUERY is sent every 90secs asking about the previous query. Three SIA-REPLY messages are allowed (It means a total of 3min). If the route is still stuck, the EIGRP device e removes the route from that neighbor and resets the adjacency with this same neighbor.
timer active time [time]Graceful shutdown is an eigrp feature to send a Goodbye message (inside hello) in the broadcast when shutdown an EIGRP process. All K’s value is set as 255.
Reliable Transport Protocol
The reliable transport is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. Note, that reliability is provided only when necessary. Only when the message must be acknowledged (type of hello). It also supports intermixed transmission of multicast and unicast packets
Example of An query and Query Exchange on multi-Access links got from RFC. Both messages request reliability:
NOTE: All update messages must be an acknowledgment from the REPLY message
Neighbor discovery/recovery
On a network with multicast capability, hello packets are sent to destination IP 224.0.0.10. There are moments that a hello packet is used to acknowledge updated messages (ACK)
Authentication
EIGRP supports two hash algorithms
MD5
. Available on both classical and named EIGRP
. Uses key chains
. supports automatic key rotation
HMAC-SHA-256 (Most secure)
. Only available on the named mode
. Static key
Tables
Like many routing protocols, EIGRP has three main tables:
EIGRP topology
show ip eigrp topology [all-links]
EIGRP Neighbors topology
show ip eigrp neighbors [detail]
Routing information base ( RIB)
show ip route eigrp
EIGRP ROUTE FILTERING
EIGRP enables route filtering using
Passive interface
passive-interface {interface | default}
Distribute list
distribute-list {ACL | Prefix-list | route-map} {in | out} {interface}
EIGRP SUMMARIZATION
Large routing tables consume more memory processing power, and more time to converge. Route summarization and route filtering can be used to reduce the size of the routing table.
. Route filtering limiting the destinations reachable;
. With route Summarization, reachability is not an issue and query scope is reduced.
EIGRP summarization can be done at any router
. Both auto and manual summarization is allowed. Auto summarization requirements:
→ Network to be advertised must be local to the router.
→ Network must be advertised using the “network” command (redistributed is not supported) (class full addresses)
Please don’t use auto-summarization
EIGRP Default route
. Option 1: by advertising a static default route with EIGRP:
→ Redistribute route into EIGRP.
→ Use network 0.0.0.0;
.Option 2: By configuring a default network
→ (config)# ip default-network x.x.x.x
.Option 3: By using the summary-address command
The only default route will be advertised;
→(config-if)#ip summary-address eigrp <asn> 0.0.0.0 0.0.0.0
Possibles problem — EIGRP Deep Dive
Tshooting eigrp adjacencies
- ACL filtering eigrp transport
→ IP 88 Unicast and multicast 224.0.0.4
2. Control Planning policy
→Mismatch policy-map with a drop action
3. Local Route based policy
4. K-value mismatch
Tshooting eigrp advertisements
- Duplicated RID
- Route filtering
- Add path and no-next-hop-self commands into the hub on the hub-and-spoke topology to advertise multiple best paths (DMPVN phase two).
- Unequal load balance only available with a route that meets feasible conditional. Change delay does reach feasibility condition and variance to get unequal load balance.
REFERENCE
RFC 7868
https://www.rfc-editor.org/rfc/inline-errata/rfc7868.html
Cisco Configuration Guide
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_eigrp/configuration/xe-16-10/ire-xe-16-10-book.html
INE CCIE Routing and Switch v5
https://ine.com
