In order to create an adjacency with the neighbor, the eigrp discovering process needs to be done first. The process of discovering a neighbor is as below:
1. The originating router sends a Hello packet using multicast to 224.0.0.10
2. The neigboring router will detect the Hello packet and responses with an Update packet using unicast (NOT multicast!). In the same time it will also send an hello packet to the originating router.
3. The originating router receives the update packet and sends an unicast Ack packet to the neighbor router while sending an update packet from its current configuration to the neighbor router in unicast.
4. The neighbor router will send an ack back to the originating router to confirm the receival of the update packet from the originating router.
EIGRP does not require the neighbors to have the same hello and hold time (the same as dead time in ospf) values which is the opposite in ospf, although this difference can create an instability in the connection between the adjacent routers. However, metric configurations (k-weights) and the AS number of the adjacent routers MUST be the same or the connection between the adjacent routers will not happen. Another difference between OSPF and EIGRP is that OSPF has a dead time = 3 x hello time whereas EIGRP has a hold time = 4 x hello time.
The hello packets are by default sent every 5 seconds on a high speed link such as Etherned and FDDI, whereas in low speed links (less than T1 speed such as ATM)
To change the hello time use the command:
(config)#interface
(config-if)#ip hello eigrp
To change the metrics weights use the command:
(config)#router eigrp // 0 is not allowed
(config-router)#metric weights 0
To make a router in an EIGRP network do the following command:
(config)#router eigrp
(config-router)#no auto summary
(config-router)#network // the network can be the primary network or the secondary network (See below)
To show the neighbors use the #show ip eigrp neighbor command
To debug the packets do #debug eigrp packets
EIGRP is also possible when the secondary addresses are applied on the routers. A secondary address is when 2 ip addresses are used on a single physical interface. However, the secondary addresses must be configured properly and the secondary subnets must be the same between the neigboring routers. If the secondary address subnet is configured on a router and the same subnet is configured on the neighboring router as primary, eigrp will not work because the primary addresses of both routers are not in the same subnet. The adjecency will be formed, but on the show ip eigrp neighbor command the adjecency is formed between the primary networks, not the secondary.
To put a secondary address on an interface do the command:
(config)#interface
(config-if)#ip address secondary
1. The originating router sends a Hello packet using multicast to 224.0.0.10
2. The neigboring router will detect the Hello packet and responses with an Update packet using unicast (NOT multicast!). In the same time it will also send an hello packet to the originating router.
3. The originating router receives the update packet and sends an unicast Ack packet to the neighbor router while sending an update packet from its current configuration to the neighbor router in unicast.
4. The neighbor router will send an ack back to the originating router to confirm the receival of the update packet from the originating router.
EIGRP does not require the neighbors to have the same hello and hold time (the same as dead time in ospf) values which is the opposite in ospf, although this difference can create an instability in the connection between the adjacent routers. However, metric configurations (k-weights) and the AS number of the adjacent routers MUST be the same or the connection between the adjacent routers will not happen. Another difference between OSPF and EIGRP is that OSPF has a dead time = 3 x hello time whereas EIGRP has a hold time = 4 x hello time.
The hello packets are by default sent every 5 seconds on a high speed link such as Etherned and FDDI, whereas in low speed links (less than T1 speed such as ATM)
To change the hello time use the command:
(config)#interface
(config-if)#ip hello eigrp
To change the metrics weights use the command:
(config)#router eigrp
(config-router)#metric weights 0
To make a router in an EIGRP network do the following command:
(config)#router eigrp
(config-router)#no auto summary
(config-router)#network
To show the neighbors use the #show ip eigrp neighbor command
To debug the packets do #debug eigrp packets
EIGRP is also possible when the secondary addresses are applied on the routers. A secondary address is when 2 ip addresses are used on a single physical interface. However, the secondary addresses must be configured properly and the secondary subnets must be the same between the neigboring routers. If the secondary address subnet is configured on a router and the same subnet is configured on the neighboring router as primary, eigrp will not work because the primary addresses of both routers are not in the same subnet. The adjecency will be formed, but on the show ip eigrp neighbor command the adjecency is formed between the primary networks, not the secondary.
To put a secondary address on an interface do the command:
(config)#interface
(config-if)#ip address
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