Jaycee's Networking

May 16, 2009

IP Routing Overview 2/2

Filed under: IOS, Static Route — Tags: — Jaycee @ 1:07 am

A. Routing Protocols:

1. EGPs are much more complicated than IGPs because they handle more routing information while performing better route summarization.

2. Distance-Vector and Link-State Routing Protocols:

a. Distance-Vector Protocols:

(1) They provide 2 information for every route: a distance (metric) and a vector (next-hop).
(2) A lower metric value means a better route.

b. Link-State Routing Protocols:

(1) They build network topology on each router and broadcast only changes to the entire network.
(2) Link-State information saves network bandwidth by reducing the amount of routing traffic needed for routing updates.

3. Administrative Distance:

a. Each route is assigned an administrative distance, based on how the route was learned.

b. Think of the route’s metric as the preference of a route, while the administrative distance is the preference of how the route was discovered.

c. A route to a network attached to a directly connected interface is the most preferred route.

B. Split Horizon:

1. Stop routing loops by telling the router NOT to advertise routes out the same interface which the route was originally learned.

=> If a router learns about a route on a particular interface, it doesn’t broadcast that route information out that interface.

2. Split horizon can’t prevent routing loops involving 3 or more routers, but it’s effective at preventing loops b/w 2 routers.

3. Split horizon is enabled by default on most interfaces.

4. Should disable split horizon on a multipoint subinterface.

5. Disable split horizon on a multipoint subinterface.

no ip split-horizon

6. Routing protocols can often work out routing loops on their own; however, split horizon solves the problem more efficiently because it prevents the loops from developing in the first place.

C. Static Routing:

1. Use interface:

ip route Etherenet0

It sends packets destined to the single host  out through the Ethernet0 interface. The router will need to figure out which device on this segment to forward the packet to because it must put the MAC address of the next-hop router in the Layer 2 frame header.

The standard mechanism for associating IP addresses with MAC address is ARP (Address Resolution Protocol). The router will send out an ARP request broadcast on the Ethernet segment.

If the device that owns the packet’s destination IP happens to be on this segment, it’ll respond with its MAC address. Otherwise, a router configured for proxy ARP will have to respond on its behalf. If you dont have proxy ARP configured on the next-hop router, this command will fail.

For multiple access media such as Ethernet segments, It’s beeter to specifying the IP address of the next-hop router rather than the interface.

2. “permanent” keyword

ip route permanent

It ensures the static route always remains in the routing table,, even if the next-hope interface is down.

There is a danger that the dynamic routing protocol will install a route that you dont want to use, so it may be preferable to drop the packets rather than to use the dynamic route.

3. Routing tags:

ip route tag 36291

Routing tags are used when redistributing from one routing protocol to another.

4. Administrative Distance value:

ip route 5

The router will use this distance value to decide between routes to the same destination prefix from different sources.

5. Floating Static Routes:

ip route 190

Router will use a floating static route for a particular network prefix ONLY IF that same route is not available from the dynamic routing protocol. It can be accomplished by setting the AD (administrative distance) of the static route to a value gureater than AD of the dynamic routing protocol.

*Remember: the router will always use the route that has the most precise match (longest netmask).

For example, if the router has learned a route for from OSPF, and also has a static route for with AD=190, it’ll use the static route even it has a higher AD.

*The AD is only used to decide between competing routes of the same mask length.

Floating static routes are often used to trigger automated backup mechanisms when the routing protocol fails.

6. Using Policy-based Routing to route based on Source address:

access-list 1 permit
access-list 2 permit
interface Ethernet0
 ip address
 ip policy route-map Engineers
 ip route-cache policy
route-map Engineers permit 10
 match ip address 1
 set ip next-hop
 set ip next-hop verify-availability
route-map Engineers permit 20
 match ip adddress 2
 set default next-hop
 (set default interface Null0)

Policy-based routing allows you to configure special routing rules beyond the normal Ip routing table.

*Every route map ends with an implicit deny all.

next-hop verify-availability” uses CDP. You have to ensure that CDP is enabled on the interface leading to this next-hop device. (That device must be another Cisco router and running CDP.)  This verification process can cause performance problems. Furthermore, CDP uses long timeout period by default (180 seconds), so it’s slow to respond to failures.

default next-hop” forces the router to discard the packets rather than using the router’s general default gateway.

ip route-cache policy” tells the router to use fast switching rather than process switching when processing policy command.

Because policy-based routing overrides the normal routing tables within the router, it can result in some confusing troubleshooting problems. Such as trying to ping from the router. The ICMP packets originating on the router will not be subject to the routing policy. So, you may find that you can ping, but that application doesn’t work for certain users.

*Recommend AVOID policy-based routing.


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