Archive for the ‘ip’ tag
Here are some packet overhead numbers for a few popular protocols to help with doing bandwidth requirement calculations. This may be another add-as-we-go post, so please comment with additions or corrections.
Ethernet : 20 bytes
Frame Relay : 4 – 6 bytes
PPP : 6 bytes
MLPPP: 10 bytes
MPLS : 4 bytes
IP : 20 bytes
TCP : 20+ bytes
UDP : 8 bytes
GRE: 4 – 20+ bytes
IPSec : 50 – 57 bytes
ESP : 20+ bytes
AH : 16+ bytes
L2TP : 24 bytes
RTP : 12 bytes
Bonus: A voice packet is always 40 bytes + data link since it will always (?) use RTP + UDP + IP.
Feel free to correct.
- What’s the most primitive way to get traffic destined to a single host to use a different path than your dynamic IGP dictates?
Use a static route.
- What’s the most primitive way to get traffic sourced from a single host to use a different path than your dynamic IGP dictates?
Use policy-based routing (PBR).
- What’s the most primitive way to get traffic sourced from a single host and destined for another host to use a different path than your dynamic IGP dictates?
- What are the steps to configure PBR?
Configure a route-map to match the desired traffic
Apply that route-map to an interface with the ip policy route-map command
- Configure PBR to send traffic that arrives on F0/0 from 10.0.0.5 destined for for 192.168.3.3 to be sent out the s0/0 interface.
R1(config)#ip access-list extended PBR-ACL1
R1(config)#permit ip host 10.0.0.5 host 192.168.3.3
R1(config-route-map)#match ip address PBR-ACL1
R1(config-route-map)#set interface s0/0
R1(config-if)#ip policy route-map PBR-F0/0
- What happens if you use PBR to redirect traffic to an IP that becomes unreachable?
That clause in the route-map is ignored, and the normal routing table is used.
- What difference does using default make in the set directive of the route-map?
If you use the default parameter in the set directive, then the router will first try to use the routing table to forward traffic before using the PBR settings. The one caveat, though, is the default chosen for the traffic cannot be the default route; a more-specific route must be in the routing table or else the PBR logic rears its head.
- What is IP SLA?
IP SLA is a feature of a Cisco IOS device where a process measures the behavior of the network.
- Why is this topic in the ROUTE book?
You can configure a track object to use IP SLAs to get a “failed” or “ok” status. That track object can be applied to static routes and PBR so that the routing is changed if the IP SLA measures a characteristic outside of normal parameters.
- What are the steps to configure IP SLA?
Create an IP SLA operation.
Define the type and parameters for the operation.
Define the frequency to run the operation.
Schedule when to start the operation.
- How do I use IP SLA to check if a host is pingable?
You use the icmp-echo as the operation type along with, at minimum, the IP address to ping.
- How can I use IP SLA to know whether a static route is usable or not?
First, create an IP SLA operation to ping the gateway for that route.
R1(config)#ip sla 5
R1(config-ip-sla)#frequency 60 [ in seconds ]
R1(config)#ip sla schedule 5 start-time now life forever
Then create a track object that references the IP SLA operation you just created.
R1(config)#track 2 ip sla 5 state
R1(config-track)#delay up 90 down 90 [ up if delay is below 90, down if above 90 ]
Finally, add the track to the static route.
R1(config)#ip route 10.0.0.0 255.255.0.0 126.96.36.199 track 2
Now, if the router can’t ping 188.8.131.52, the static route will be taken out of the routing table.
- What’s an IP SLA responder?
That’s (usually) a router that has been configured to interact with the IP SLA operation of another router to get characteristics of the connection between the two. These characteristics include jitter and TCP establishment times.
- How can I use a track object in PBR?
In the set directive, you use the track parameter. The sequence parameter is also used, but it’s not a part of the tracking process; it’s used to have the router go down a list of next hops until it finds on that’s available. Here’s an example.
set ip next-hop verify-availability 192.168.0.1 1 track 5
- Ummm…the book doesn’t have anything about that; what gives?
The cert guide leaves that part out for some reason even though it’s a very important part of IP SLA and PBR. Go figure.
What Command Was That
- …shows interfaces that have PBR configured on them?
show ip policy
- …shows the routing table and includes all the PBR configuration?
There isn’t one. You have to remember to check for PBR when traffic isn’t flowing as you think it should.
- …shows the IP SLA configuration?
show ip sla configuration [ Duh! ]
- …shows the IP SLA statistics?
show ip sla statistics [ Duh, again! ]
- …shows the track objects on a router?
I’ve seen and used the command before, but I’ve never really seen any use of the show ip protocols command until tonight while reading up for my ROUTE test. There’s a lot of good information in the output, and, from the way the book is reading, this is a great candidate for use in a lab question.
To check it out a bit, I set up a small network with four routers connected only to a single Ethernet segment. I set up one router to run EIGRP, OSPF, and BGP to each one of the other routers just so I could see the output for the different routing protocols. Here’s what puked out after struggling with GNS for a few minutes.
R1#sh ip protocols Routing Protocol is "eigrp 1" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Default networks flagged in outgoing updates Default networks accepted from incoming updates EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 EIGRP maximum hopcount 100 EIGRP maximum metric variance 1 Redistributing: eigrp 1 EIGRP NSF-aware route hold timer is 240s Automatic network summarization is in effect Maximum path: 4 Routing for Networks: 192.168.0.0 Routing Information Sources: Gateway Distance Last Update Distance: internal 90 external 170 Routing Protocol is "ospf 1" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Router ID 192.168.0.101 Number of areas in this router is 1. 1 normal 0 stub 0 nssa Maximum path: 4 Routing for Networks: 192.168.0.0 0.0.0.255 area 0 Reference bandwidth unit is 100 mbps Routing Information Sources: Gateway Distance Last Update Distance: (default is 110) Routing Protocol is "bgp 65001" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set IGP synchronization is disabled Automatic route summarization is disabled Neighbor(s): Address FiltIn FiltOut DistIn DistOut Weight RouteMap 192.168.0.104 Maximum path: 1 Routing Information Sources: Gateway Distance Last Update Distance: external 20 internal 200 local 200
The EIGRP section shows some important details, including what k-values are used, networks configured, and administrative distance (AD) of the various route types (internal and external). The OSPF section shows the router ID, number of areas on the router, and number of area types (normal, stub, NSSA), as well as the networks configured and the AD. The section regarding BGP shows summarization status, neighbors (along with any filter lists, distribution lists, local weights, and route-maps if they were configured), and the ADs again.
That’s good stuff to know. I’ll have to put that command in usual repertoire.